JP3302477B2 - Surface acoustic wave filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a filter using a surface acoustic wave, and more particularly to an element structure of a surface acoustic wave filter used as a front end filter in a receiving section of a mobile communication device or the like.

[0002]

2. Description of the Related Art A front-end filter used in a receiving unit of a mobile communication device or the like is used to select a received signal before frequency conversion. The front-end filter has a low insertion loss and a required band can be obtained without ripple. It is desirable that As a filter suitable for such use, IIDT
There is a surface acoustic wave filter having an electrode structure. The IIDT electrode structure is such that a number of input and output IDTs (comb-shaped electrodes) are alternately arranged and connected in parallel, and there is no loss of directionality in the transversal-type filter. A relatively wide pass band can be obtained as compared with the type filter.

[0003]

The characteristics of the conventional surface acoustic wave filter are not always sufficient as a front-end filter used for mobile communication from the following points.

[0004] The amplitude ripple in the pass band is large.
In particular, when the metal film forming the electrode finger is thick, the reflectance by the electrode finger increases, and the amplitude ripple increases. In addition,
In the use in the UHF band (300 MHz) or higher, if the metal film thickness is reduced in order to reduce the reflectance, the internal resistance of the electrode finger increases and the insertion loss increases.

An external matching circuit is required. Normally, a pure resistance value is used as the input / output impedance of the front end filter. However, the matching condition for obtaining the optimum characteristics of the surface acoustic wave filter is not limited to the pure resistance, and therefore, it is necessary to add an external matching circuit. In particular, in mobile communication equipment where miniaturization of equipment is important,
This leads to an increase in the number of components and the mounting area, which is a major problem. Also, the pass bandwidth may not be sufficient for some applications.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a surface acoustic wave filter which has sufficient characteristics as a front end filter for mobile communication, that is, a small, low insertion loss, and a low ripple in a pass band, capable of obtaining a required pass bandwidth. Is provided.

[0007]

A surface acoustic wave filter according to the present invention comprises: (a) a substrate made of a piezoelectric material; and (b) a plurality of comb-shaped filters arranged on the surface of the substrate and connected in parallel with each other. A first comb-shaped electrode group consisting of electrodes;
A second interdigital electrode group comprising a plurality of interdigital electrodes arranged on the substrate surface between each interdigital electrode of the interdigital electrode group and connected in parallel with each other; and (d) The distance L1 between the electrode finger centers of the comb electrodes belonging to the first comb electrode group and the adjacent comb electrodes belonging to the second comb electrode group adjacent thereto is

[Equation 1] (0.30 + n / 2) λ ≦ L1 ≦ (0.45+
n / 2) λ (where λ is the period of the comb-shaped electrode, n is zero or a natural number), and (e) a comb included in at least one of the first and second comb-shaped electrode groups. The logarithm of the mold electrode and the electrode film thickness are set to values at which the radiation conductance of the comb electrode group within the passband frequency is larger than the absolute value of the radiation susceptance.

In addition, (f) a reflector consisting of a plurality of metal strips arranged so as to sandwich the first and second comb-shaped electrode groups and short-circuited to each other at the same period as the electrode fingers of the comb-shaped electrode. (G) the center distance L2 between the center of the metal strip closest to the comb-shaped electrode and the center of the electrode finger of the adjacent comb-shaped electrode is

(Equation 2) (0.35 + m / 2) λ ≦ L2 ≦ (0.65+
m / 2) λ (where λ is the period of the comb-shaped electrode and m is zero or a natural number).

Further, the logarithm and the electrode film thickness of all the comb electrodes included in at least one of the first and second comb electrode groups are such that the radiation conductance of the comb electrodes within the passband frequency radiates. It is desirable that the value is set to a value larger than the absolute value of the susceptance. More specifically, when the substrate is a 30-40 ° Y-rotation LiTaO
₃ , and the logarithm N of the comb-shaped electrode and the electrode thickness h are

[Equation 3] 15 + log ₁₀ (h / λ) ≦ N ≦ −20-40 log
₁₀ (h / λ) (where λ is the period of the comb-shaped electrode, and log ₁₀ (x) is a common logarithm of x).

[0010]

FIG. 1 schematically shows a first electrode configuration according to the present invention. A plurality of input comb electrodes 2 and a plurality of output comb electrodes 3 are alternately arranged adjacent to each other on the surface of a substrate 1 made of a piezoelectric material. One of the input comb electrode 2 and the output comb electrode 3 are connected in parallel by the parallel wirings 4 and 5, and the other is grounded. The distance between the input comb electrode 2 and the adjacent output comb electrode 3 is determined by the distance L1 between the centers of the closest electrode fingers 6 belonging to each.

[Equation 1] (0.30 + n / 2) λ ≦ L1 ≦ (0.45+
n / 2) λ (where λ is the period of the comb-shaped electrode, and n is zero or a natural number). The radiation conductance in the passband frequency of the entire input comb electrode 2 connected in parallel (and / or the entire output comb electrode 3 connected in parallel) is the absolute value of the radiation susceptance. The logarithm and electrode thickness of the input comb-shaped electrode 2 (and / or the output comb-shaped electrode 3) are set so as to be larger (preferably three times or more).

FIG. 2 shows the pass characteristics (pass band width is indicated by +, insertion loss is indicated by ●, and in-band ripple is indicated by ○) when the center distance L 1 between the electrode fingers is changed in such an electrode configuration. Put together. It can be seen that the insertion loss is minimized in the vicinity of 0.87λ, and is particularly low at 0.75 to 0.95λ. At 0.87 to 1.1λ, no in-band ripple occurs, and at 0.80 to 0.87λ, the in-band ripple can be reduced to 1.0 dB or less. Further, it can be seen that a relatively wide pass bandwidth of about 3% can be obtained in this range. Therefore, the distance L1 between the centers of the electrode fingers is set to 0.8.
By setting to 0 to 0.95λ, the insertion loss is low,
In addition, a filter having a low in-band ripple can be configured. In particular, when the normalized film thickness is relatively thick (3% or more), the ripple is low without using an external matching circuit,
In addition, the insertion loss can be reduced, and in addition, the electrical resistance of the electrode finger can be reduced, and the insertion loss due to the internal resistance of the electrode can be further reduced. This characteristic has a periodicity of λ / 2, and the distance L1 between the centers of the electrode fingers is set to 1.30 to
It may be set to 1.45λ or the like, and it can be seen that similar characteristics can be obtained by changing at a period of λ / 2. It should be noted that this characteristic is obtained by inputting 5 input signals on a 36 ° Y-rotation LiTaO ₃ substrate.
This is a simulation of a passing characteristic when five sets of comb electrodes (25.5 pairs) are alternately arranged and the input / output impedance is set to 50Ω.

FIG. 3 schematically shows a second electrode configuration according to the present invention.
Shown in This electrode structure is arranged so as to sandwich the input comb electrode 2 and the output comb electrode 3 in addition to the first electrode structure, and has the same period (λ / 2) as the electrode fingers 6 of these comb electrodes. Metal strips 7 shorted to each other
And a center of the metal strip 7 closest to the comb electrodes 2 and 3 of the reflector 8;
The distance L2 between the center of the electrode finger 6 of the adjacent comb-shaped electrode and the center L2 is

(Equation 2) (0.35 + m / 2) λ ≦ L2 ≦ (0.65+
m / 2) λ (where λ is the period of the comb-shaped electrode, and m is zero or a natural number).

In such an electrode configuration, the pass band width when the center distance L2 between the metal strip 7 and the electrode finger 6 of the interdigital electrode is changed (the 3 dB bandwidth is indicated by ○, and the 1 dB bandwidth is indicated by ●). Are summarized in FIG. Center distance L2
Is about 0.75λ, the bandwidth becomes extremely small (3 dB bandwidth is about 1.2%), and its passing characteristic causes a large ripple in the band. 0.65λ or less or 0.85
Above λ, the 3 dB bandwidth becomes 2.5% or more, which can be sufficiently widened. 0.60λ or less or 0.95λ
By doing so, the pass bandwidth can be further increased. These properties are based on the 36 ° Y-rotation LiTaO ₃
Five sets of input and five sets of comb-shaped electrodes (25.
5 pairs) and a reflector composed of 40 metal strips short-circuited to each other, and the center distance L1 between the electrode fingers is set to 0.
This is a simulation of the passing characteristic when the input / output impedance is 85Ω and the input / output impedance is 50Ω.

[0014] Insertion in the case where the number (the number of sets) of comb-shaped electrodes for input and output is changed using the first electrode configuration without a reflector and the second electrode configuration with a reflector. The change in loss is shown in FIG. It can be seen that the use of the second electrode configuration with a reflector reduces insertion loss as compared to the first electrode configuration without a reflector. In particular, when seven or more comb-shaped electrodes are used, it can be seen that the insertion loss becomes sufficiently low.

Further, in the first and second electrode structures, the logarithm of the comb-shaped electrode 2 and / or the electrode thickness of the comb-shaped output electrode 3 and the electrode film thickness are determined by the radiation conductance within the pass band frequency. Is set to a value larger than the absolute value of the radiation susceptance, it is possible to match the external impedance of the pure resistance without providing an external matching circuit. Therefore, it is possible to reduce the size of the entire circuit. In particular, when a 36 ° Y-rotation LiTaO ₃ substrate having a high surface acoustic wave velocity and excellent temperature characteristics is used, the logarithm N of the comb-shaped electrode and the electrode thickness h are reduced.

[Equation 3] 15 + log ₁₀ (h / λ) ≦ N ≦ −20-40 log
₁₀ (h / λ) (where λ is the period of the comb-shaped electrode and log ₁₀ (x) is a common logarithm of x) so that the input comb-shaped electrode 2 and / or the output Since the radiation conductance of the comb-shaped electrode 3 can be made smaller than the absolute value of the radiation susceptance, matching is easy and the device can be downsized. FIG. 6 shows a 36 ° Y-rotation LiTaO.
The range of the logarithm in one possible comb-shaped electrode that reduces the absolute value of the radiation susceptance when _three substrates are used is shown. It can be seen that when the normalized film thickness h / λ is increased (especially 4% or more), the range of the logarithm becomes narrow (about 18 to 25 pairs), and the range of Expression 3 is shown.

As described above, according to the present invention, it is possible to realize a small surface acoustic wave filter having a sufficiently wide pass band, a low insertion loss, a small ripple in the pass band, and a small surface acoustic wave filter. Note that the above simulation results
The result is similar to the actual measurement. 36 ° Y rotation LiTa
The O ₃ substrate has substantially the same characteristics when the rotation angle is in the range of 30 to 40 ° and when the propagation direction of the surface acoustic wave is in the X direction or within 5 °.

[0017]

The present invention will be described below in detail with reference to examples. The outline of the electrode structure of the SAW filter according to the first embodiment is the same as that shown in FIG. L of 36 ° Y rotation
The substrate 1 made of iTaO ₃ (single crystal of lithium tantalate) was used, and the propagation direction of the surface acoustic wave was set to the X direction. On the surface of the substrate 1, five input comb electrodes 2 and five output comb electrodes 3 are alternately arranged adjacent to each other. One of the comb electrode 2 for input and the comb electrode 3 for output are connected in parallel by the wirings 4 and 5 for parallel connection, and the other is grounded. The comb electrodes 2 and 3 and the wirings 4 and 5 are made of an aluminum film having a normalized film thickness h / λ of 4.5%. These comb electrodes 2 and 3 have a logarithm of 25.
5 pairs, a single electrode (the width of the electrode finger and the interval between the electrode fingers are each approximately λ / 4), and the aperture length is 20
λ. The distance L1 between the centers of the closest electrode fingers 6 belonging to each of the input comb electrode 2 and the adjacent output comb electrode 3 was set to 0.8λ. Note that, in the pass frequency band, the absolute value of the radiation conductance of one of the comb electrodes 3 and 4 is at least three times the absolute value of the radiation susceptance.

The input / output terminating impedance is 50Ω + j
SAW according to the first embodiment when 0Ω (pure resistance) is set
The pass characteristic of the filter (frequency f is the pass center frequency fo
FIG. 7 shows the normalized frequency (f / fo) standardized in (1). 1
Good filter characteristics with a dB ratio bandwidth of 2% or more, a minimum insertion loss of 2 dB or less, and an in-band ripple of 0.5 dB or less were obtained. The pass band of the filter is within the stop band of the comb-shaped electrode determined by the reflection of the surface acoustic wave, and resonates in a single mode, so that excellent characteristics without in-band ripple can be obtained.

As Comparative Example 1, the distance L between the centers of the electrode fingers 6
FIG. 8 shows the pass characteristics of a filter having the same value as in the first embodiment except that 1 is set to 1.0λ. Only a narrow filter characteristic with a 1 dB ratio bandwidth of less than 1% can be obtained, and in addition, the attenuation outside the pass band is not sufficiently obtained.

Next, the SA having the electrode structure shown in FIG.
A W filter will be described as a second embodiment. In the second embodiment, a substrate 1, an input comb electrode 2, an output comb electrode 3, and wirings 4 and 5 similar to the first embodiment are provided. The distance L1 between the centers of the closest electrode fingers belonging to each of the comb electrodes 3 is 0.8λ. Further, it is arranged so as to sandwich the input comb electrode 2 and the output comb electrode 3 and has the same period as the electrode fingers 6 of these comb electrodes (a metal part of λ / 4,
(having a repetition of a metal-free part of λ / 4) comprising a pair of reflectors 8 of forty metal strips 7 shorted together. (The metal strip 7 is made of the same aluminum film as the interdigital electrodes 3 and 4.) Reflector 8
The center distance L2 between the center of the metal strip 7 closest to the comb electrodes 3 and 4 and the electrode finger 6 of the adjacent comb electrode is 0.5λ.

The terminating impedance of the input and output is 50Ω + j
SAW according to the second embodiment when 0Ω (pure resistance) is set
FIG. 9 shows the pass characteristics of the filter. 1dB ratio bandwidth 2%
As described above, good filter characteristics with a minimum insertion loss of 1.5 dB or less and an in-band ripple of 0.5 dB or less were obtained. It can be seen that the insertion loss is improved by 0.4 dB or more compared to the first embodiment by using the reflector 8.

As a comparative example 2, the pass characteristics of the center of the metal strip 7 and the center distance L2 between the electrode fingers 6 of the comb-shaped electrode adjacent to the center were set to 0.75λ, and otherwise the same as that of the second embodiment. As shown in FIG. Only a filter characteristic having a large ripple (5 dB or more) in the band can be obtained.

Note that the present invention is not limited to the above-described embodiment, and a plurality of filters according to the present invention may be connected in cascade as an electrode configuration. The input / output terminals are unbalanced by grounding one side, but may be of a balanced input / output type without grounding. For the comb electrode, copper, silicon, or the like may be added to aluminum, or another conductive material may be used. Also, the number of comb-shaped electrodes for input and the number of comb-shaped electrodes for output may not be the same (N: N), but one may be increased by one (N: N + 1). Easy impedance matching conditions. In particular, it is desirable from the viewpoint of filter characteristics that the electrode structure, including the number of comb electrodes, is designed to be point-symmetrical.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram for explaining a first electrode configuration according to the present invention.

FIG. 2 is a diagram illustrating transmission characteristics when the distance L1 between centers of electrode fingers is changed in the first electrode configuration.

FIG. 3 is a conceptual diagram for explaining a second electrode configuration according to the present invention.

FIG. 4 is a diagram showing a pass band width when a center distance L2 between a metal strip and an electrode finger is changed in the second electrode configuration.

FIG. 5 is a diagram showing insertion loss when the number of comb electrodes is changed in the first and second electrode configurations.

FIG. 6 is a diagram showing a range of a logarithm that can reduce an absolute value of a radiation susceptance when a LiTaO ₃ substrate is used.

FIG. 7 is a diagram illustrating pass characteristics of the SAW filter according to the first embodiment.

FIG. 8 is a diagram illustrating pass characteristics of a SAW filter according to a first comparative example.

FIG. 9 is a diagram illustrating pass characteristics of a SAW filter according to a second embodiment.

FIG. 10 is a diagram illustrating pass characteristics of a SAW filter according to a second comparative example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate 2 Comb electrode for input 3 Comb electrode for output 4, 5 Parallel wiring 6 Electrode finger 7 Metal strip 8 Reflector

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-6-69752 (JP, A) JP-A-3-284009 (JP, A) JP-A-7-38369 (JP, A) JP-A-3- 91311 (JP, A) Hideo Onuki, Norio Hosaka, Jun Yamada, An Optimization Method in Designing Multi-electrode Low-Loss SAW Filters for Mobile Communications, IEICE Transactions A, February 25, 1993, p . 167-174 (58) Field surveyed (Int. Cl. ⁷ , DB name) H03H 9/145 H03H 9/64

Claims (4)

(57) [Claims] (A) a substrate made of a piezoelectric material; (b) a first comb-shaped electrode group consisting of a plurality of comb-shaped electrodes arranged on the surface of the substrate and connected in parallel with each other; c) the first
A second interdigital electrode group comprising a plurality of interdigital electrodes arranged on the substrate surface between each interdigital electrode of the interdigital electrode group and connected in parallel with each other; and (d) The distance L1 between the electrode finger centers of the comb-shaped electrodes belonging to the first comb-shaped electrode group and the adjacent comb-shaped electrodes belonging to the second comb-shaped electrode group is expressed by the following formula: (0.30 + n / 2) λ ≦ L1 ≦ (0.45+
n / 2) λ (where λ is the period of the comb-shaped electrode, n is zero or a natural number), and (e) a comb included in at least one of the first and second comb-shaped electrode groups. A surface acoustic wave filter characterized in that the logarithm and the electrode thickness of the pattern electrode are set to values such that the radiation conductance of the comb electrode group within the pass band frequency is larger than the absolute value of the radiation susceptance. And (f) a reflector comprising a plurality of metal strips which are arranged so as to sandwich said first and second comb-shaped electrode groups and are mutually short-circuited at the same period as the electrode fingers of said comb-shaped electrode. (G) the center-to-center distance L2 between the center of the metal strip closest to the comb-shaped electrode and the center of the electrode finger of the adjacent comb-shaped electrode is expressed by the following equation (0.35 + m / 2) λ ≦ L2 ≦ (0.65+
2. The surface acoustic wave filter according to claim 1, wherein m / 2) λ (where λ is the period of the comb-shaped electrode, and m is zero or a natural number). 3. The logarithm and the electrode film thickness of all the comb electrodes included in at least one of the first and second comb electrode groups are such that the radiation conductance of the comb electrodes within the pass band frequency radiates. 3. The surface acoustic wave filter according to claim 1, wherein the value is set to a value larger than the absolute value of the susceptance. 4. The method according to claim 1, wherein the substrate is a 30-40 ° Y rotation LiTaO.
₃ , and the logarithm N of the comb-shaped electrode and the electrode thickness h are as follows: 15 + log ₁₀ (h / λ) ≦ N ≦ −20-40 log
4. The surface acoustic wave filter according to claim 3, wherein the filter satisfies ₁₀ (h / λ) (where λ is the period of the comb-shaped electrode and log ₁₀ (x) is a common logarithm of x).